download-rustc: Give a better error message if artifacts can't be downloaded

Before:
```
downloading https://ci-artifacts.rust-lang.org/rustc-builds/bf5cad8e775fb326465e5c1b98693e5d259da156/rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz
curl: (22) The requested URL returned error: 404
```

After:
```
downloading https://ci-artifacts.rust-lang.org/rustc-builds/bf5cad8e775fb326465e5c1b98693e5d259da156/rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz
curl: (22) The requested URL returned error: 404

error: failed to download pre-built rustc from CI

note: old builds get deleted after a certain time
help: if trying to compile an old commit of rustc, disable `download-rustc` in config.toml:

[rust]
download-rustc = false
```

Author: jyn514

src/bootstrap/download.rs

@@ -541,7 +541,18 @@ impl Config {
             None
         };
 
-        self.download_file(&format!("{base_url}/{url}"), &tarball, "");
+        let mut help_on_error = "";
+        if destination == "ci-rustc" {
+            help_on_error = "error: failed to download pre-built rustc from CI
+
+note: old builds get deleted after a certain time
+help: if trying to compile an old commit of rustc, disable `download-rustc` in config.toml:
+
+[rust]
+download-rustc = false
+";
+        }
+        self.download_file(&format!("{base_url}/{url}"), &tarball, help_on_error);
         if let Some(sha256) = checksum {
             if !self.verify(&tarball, sha256) {
                 panic!("failed to verify {}", tarball.display());